The invention relates to devices used to facilitate fire-resistant, pressure-tight penetrations of decks, bulkheads, walls, floors, and similar partitions. More particularly, the invention relates to a novel elastomeric module for use in fire-rated electrical cable penetration seal devices and which provides improved pressure sealing and closer conformance to the outside surface of a cable, while maintaining the fire resistance of traditional modules. It will be understood that the apparatus in accordance with the invention must function in a demanding ambient. Thus, it will be apparent that prior apparatus that uses soft gels to cushion the connection of two wires could not possibly sustain the pressure required much less survive a fire and continue to function as the present invention is required to do.
Although the invention will be described primarily in terms of electrical power, control and communications cables it will be understood that the same apparatus will also have application to other penetrating elements. Known seal devices are described in the Brattberg U.S. Pat. Nos. 2,732,226; and 3,282,544 and other patents, and are generally known as through-penetration firestop devices or transits.
Typically these devices comprise a plurality of rubber modules. Each module is ordinarily designed to fit closely around one cable. The modules are retained in a frame which is attached to the deck, bulkhead, wall, floor, or similar partition. The frame is most commonly steel, but may be manufactured of other materials. Each rubber module is typically molded in two halves. Each half is commonly symmetrical about a plane extending through the axis of both the cable and the module. The halves form a block with a cylindrical center passage. The modules are molded in various sizes to cover the range of cable diameters. The rubber is typically controlled to a durometer hardness range of 65 to 80, to ensure mechanical stability when subjected to high pressures. Accessory items are furnished to anchor rows of modules in place, and to close the final opening in the frame while putting the modules in compression.
Transit devices or systems are used whenever there is a fairly large cable population to be accommodated in a limited space. For example, naval ships are divided into compartments by water-tight, fire-resistant bulkheads as protection in the event of damage to their hulls, such as by mines, torpedoes, or submerged objects. A pressure-tight, fire resistant seal is required to permit electrical cables to enter or pass through these compartments.
There are several factors that influence the quality of the fit between a module and the cable disposed in that module. Obviously, the fit is dependent, in part, on the compression and retention of the assembly of modules in the supporting frame. The present invention is directed to accommodating the variations in cables. Cables usually are not perfectly round or straight, and their diameters are neither uniform nor exact. Since modules must be molded in discrete sizes, a perfect fit is improbable. In the past, cable jackets have been relatively soft and compliant. The rubber module and cable jacket have been capable of mutual deformation, so that a close fit develops when the system is put in compression.
Improvements in cable jacket and insulation materials in recent years have concentrated on fire performance. New materials have been introduced with low flame spread and smoke emission characteristics. Cable jackets made with these materials are significantly harder and more rigid than earlier PVC jackets. The stiffness of the new jacket materials has made it more difficult to obtain a close fit between a module and a cable. While it might appear that reducing the hardness of the module would produce a compensating ability to conform to cable variations, this approach has proved impractical. When the modules are made sufficiently soft, the column strength of the rubber mass under compression is too low for stability, resulting in excessive deformation of the modules and leakage at lower pressures.
One solution to this problem is to provide modules with interior grooves or contours, creating multiple diameters. The effect of this design is to reduce the bearing surface of the module on cable, increasing the force available to produce mutual deformation. In this way it is possible to maintain good pressure tightness, but at the price of much higher local stress concentration on the cable jacket. The long term effect on cable jacket performance is not known, but believed to be adverse.
Another solution is to devise a module with layers that can be peeled off to tailor the groove diameter to the specific cable dimension. This design has the advantage of minimizing inventory requirements, but puts heavy responsibility on the installer to remove the correct number of layers, and creates a cleanup and disposal problem as the individual layers are discarded.
It is an object of the invention to provide a more intimate fit in a through-penetration firestop device or transit between the individual modules and the respective cables they enclose.
A related object of the invention is to improve the fluid pressure seal and thus reduce passage of noxious or explosive fluids and particularly gases between compartments even if the cable jacket is of unusual or irregular shape or design.
It is an object of the invention to provide apparatus which is inexpensive to manufacture as well as requires a minimum of labor to install.
Still another object of the invention is to provide an optimum combination of the advantages of the low durometer elastomer to conform to variations in cable jacket shape or dimension with a high durometer elastomer for mechanical strength and stability.
Yet another object of the invention is to accommodate a wider range of cable diameters than would be possible with ordinary modules of prior design, thus reducing both inventory and tooling requirements.
Yet another object of the invention is to simplify installation requirements by accommodating non-compliant or irregularly shaped penetrating items.
It is also an object of the invention to provide a structure which, when exposed to fire conditions, expands to close apertures caused by burning away of cable jackets.
Another object of the invention is to minimize cable jacket compression that could lower the dielectric strength of the cable jacket material.